The present invention relates in general to regulating the phase currents in a multi-phase motor, and more specifically to sensing the phase currents in a multi-phase motor using a single voltage comparator.
Variable reluctance motors have teeth-like poles on both the stator and the rotor. There are windings on the stator but no windings on the rotor. Each pair of diametrically opposite stator windings is connected in series to form one phase of the motor.
Torque is produced by switching current on in the phases in a predetermined sequence so that a magnetic force of attraction results between the rotor and stator poles that are approaching each other. The current is switched off in each pair of windings at the commutation point before the rotor poles nearest the stator poles of that pair of windings rotate past the aligned position, otherwise the magnetic force of attraction will produce a negative or braking torque.
The torque developed is independent of current direction. Therefore, unidirectional current pulses synchronized with rotor movement can be generated in a converter using a single unidirectional current switching element such as a thyristor or transistor in each leg of the converter, and supplied to the corresponding phase of the motor.
Each time a phase of the motor is energized by closing a switch in the converter, current flows in the pair of stator windings of that phase, providing energy from a DC supply to the motor. The energy drawn from the supply is converted partly into mechanical energy, by causing the rotor to rotate towards a minimum reluctance configuration, and converted partly into stored energy in the magnetic field. When the switch is opened, part of the stored magnetic energy is converted to mechanical output and the remainder of the energy is preferably returned to the DC supply.
The motor may be run open-loop as in a variable reluctance stepping motor, or may be run closed-loop as in a switched reluctance motor. Furthermore, the motor may be operated such that no two phases are conducting simultaneously (i.e. nonoverlapping conduction intervals) or such that some phases do conduct simultaneously (i.e. overlapping conduction intervals).
Current regulators are employed for limiting the phase current amplitudes in the motor. The current level in each phase is sensed by individual current shunts connected to each phase winding. The output of each current shunt is connected to a separate voltage comparator. Each comparator is also connected to a separate potentiometer for setting the current limit. In order to adjust all of the phase currents simultaneously using a single control knob, a multi-stage ganged potentiometer is used.
By employing separate current shunts in each phase, the power loss in the shunts is high. This is particularly true in automotive applications where the low 12 volt source voltage magnifies the efficiency penalty associated with the resistive shunt voltage drops. The necessity of a separate comparator for each motor phase inflates the parts count and the expense of the regulator. A further disadvantage is that the multi-stage ganged potentiometer is expensive.
Accordingly, it is a principal object of the present invention to provide a current regulator having reduced power loss and increased efficiency.
It is a further object of the present invention to provide a current regulator having a reduced parts count.
It is yet another object of the invention to eliminate the need for a multi-stage ganged potentiometer in a current regulator for a multi-phase motor.